Autoimmune thyroid diseases (AITD) are known to be clustered in families, but to what extent this occurs in childhood and adolescence is not well defined. In order to establish the prevalence of AITD in the siblings of affected children and adolescents, we examined 73 siblings from 66 families selected on the basis of a pediatric index patient. Sixty-six families, including a total of 146 offspring, were selected on the basis of diagnosis of chronic lymphocytic thyroiditis (CLT) (n = 55) or Graves' disease (GD) (n = 11). Among the 73 siblings examined, 20 new cases of CLT (27%) were detected. L-Thyroxine therapy was required in 4/20. History of AITD was recorded in 24/66 mothers (36%), and in two fathers. Overall in these families, considering both the index patients and the new patients, 86/141 (61%) children and adolescents were affected with AITD, with a female/male ratio of 3.3:1. Our study confirms that AITD clusters in families with a high prevalence in the siblings of affected children and adolescents. These children should be followed in order to avoid undiagnosed hypothyroidism. Prospective studies are warranted to identify predictive factors for overt thyroid disease.